In a cab signal speed command system of a transit operation, it is an authoritative requirement that the overspeed equipment must be capable of functioning over a wide range of supply voltages. Theoretically, the main car battery may vary between 22 to 44 volts d.c. In practice, the car-carried apparatus operates on 28 volts d.c., and the nominal battery voltage is 37 volts d.c. It has been found that the battery voltage seldom drops below 37 volts so that a simple series regulator may generally be used to provide the necessary 28 volts d.c. most of the time. However, even though the battery supply voltage falls below 37 volts only two percent (2%) of the time, it is necessary to employ a suitable device for maintaining the 28 volts at all times. In the past, a continuous operating d.c. to d.c. converter power supply was relied on to produce the required operating potential. However, most conventional converter circuits are costly to maintain or unreliable in continuous operation. Accordingly, there is a need for providing a power supply which is reliable in operation and inexpensive to maintain.